Present commercial operations for hermetically sealing open-ended packaging containers, especially food containers, generally employ either (1) a conventional metallic end, with or without an easy-opening feature, which is double-seamed to a flange on the container, or (2) a film or foil lidding structure, which is attached by adhesive, thermal or other bonding means to a flange on the open end of the container.
With the increasing demand for convenience packaging and ready-to-eat food products, a growing need exists for shelf-stable microwavable food packages. A significant number of these are semi-rigid plastic packaging containers. Food packages for these applications are generally required to undergo a retort/sterilization process to insure that the product is sterile and safe for consumption. Because of the high temperatures and internal pressures associated with the retort process, it is an extremely critical part of the packaging operation to provide a packaging container and closure therefor which have great strength and integrity.
Conventional double-seamed metal ends or closures employed to seal these containers provide seal integrity throughout the retort process, and can provide a convenient easy-open feature for the consumer. The metal end, however, is not entirely appropriate for a package designed to be reheated in a microwave oven, due to arcing and localized heating which can occur during heating. The metal end is also generally not suitable for very wide containers, such as bowls or trays, for a variety of reasons, including the high cost of such an end, and the aesthetically negative appearance of such a package. Additionally, the metal end cannot be used at all in those instances where the heating method of retort comes from microwave energy, due to the very large metallic surface areas which would reflect energy and defeat the ability of the microwave energy to heat the package contents. Ready-to-eat packages which employ a metal end often have a sharp metal edge exposed after removal of the easy-open lid, which presents a possible consumer safety hazard.
Film or foil lidding systems as applied to retortable packages are the most sensitive materials used in the thermal processing. The adhesive or heat-sealed bond between the body of the packaging container and the lidding material is very fragile during the heat processing period. The bond strength can be reduced by as much as 80% at elevated temperatures. Pressures inside the sealed container can rise dramatically during heating, caused by expansion of the product and gases, which place further demands on the sealing area. An additional challenge to the processor is that many of the film/foil lidding systems are designed to have peelable seals to allow for easy opening. An improved type of flexible lidding systems is disclosed, for example, in U.S. Pat. Nos. 4,801,041; 4,858,780; 4,905,838 and 4,913,307, assigned to Idemitsu Petrochemical Co., Ltd. and commercialized as "MAGICTOP" and which overcomes some of the problems with prior flexible lidding systems. However, all of these flexible lidding systems are attached by bonding to the open end of the container and most processors who employ these types of flexible lidding system must perform extensive testing of seal integrity, including 100% inspection, and require very tight controls on retort conditions to insure against seal failure and/or spoilage of contents.
An improved flexible lidding system in the form of an easy-opening closure for hermetic sealing of an open end of a packaging container has been disclosed in U.S. Pat. No. 5,069,355 and is disclosed in pending U.S. Patent application, Ser. No. 07/684,161 and filed Apr. 11, 1991, both of which are assigned to the assignee of the present invention. In this issued patent and the pending application, a composite end member and a composite lidding member, respectively, are disclosed which respectively utilize two polymer layers defining a predetermined peel strength at an interfacial plane thereof for controlled delamination of the two layers. The end member is of desired construction for being double-seamed to the open end of the container and has an interior ledge defining a large central opening to allow access therethrough to the interior of the container. The lidding member is of desired configuration for covering the central opening and has a portion overlapping the ledge portion and is bonded to the ledge portion to define a predetermined bond strength greater than the peel strength of the two polymer layers. The closure is easily opened by upward pulling of the bonded overlapping portion of the lidding member to cause delamination and peeling of the top polymer layer of the composite end member bonded to the lidding member from the bottom polymer layer in the closure construction of U.S. Pat. No. 5,069,355 and to cause delamination and peeling of the portion of the top polymer layer from the bottom polymer layer in the composite lidding member bonded to the end member in pending U.S. Patent application Ser. No. 07/684,161. In both of these systems, although the peel strength of the two polymer layers of the respective lidding member or end member are greater than the bond strength of that member to the other of the members, the force required to open the container is not consistent since the containers are opened linearly and a greater area must be peeled at the beginning and at the end of the opening operation than in the intermediate part of the opening operation, as will be explained more fully below.